Cassini watches storm choke on its own tail

In a new paper that provides the most detail yet about the life and death of a monstrous thunder-and-lightning storm on Saturn, scientists from Nasa’s Cassini mission describe how the massive storm churned around the planet until it encountered its own tail and sputtered out. It is the first time scientists have observed a storm consume itself in this way anywhere in the solar system.

“This Saturn storm behaved like a terrestrial hurricane – but with a twist unique to Saturn,” said Andrew Ingersoll, a Cassini imaging team member based at the California Institute of Technology, Pasadena, who is a co-author on the new paper in the journal Icarus.

Earth’s hurricanes feed off the energy of warm water and leave a cold-water wake. This storm in Saturn’s northern hemisphere also feasted off warm “air” in the gas giant’s atmosphere. The storm, first detected on 5 December 2010, and tracked by Cassini’s radio and plasma wave subsystem and imaging cameras, erupted around 33 degrees north latitude. Shortly after the bright, turbulent head of the storm emerged and started moving west, it spawned a clockwise-spinning vortex that drifted much more slowly. Within months, the storm wrapped around the planet at that latitude, stretching about 300 000 kilometres in circumference, thundering and throwing lightning along the way.

Terrestrial storms have never run into their own wakes – they encounter topographic features like mountains first and expend themselves. But Saturn has no land to stop its hurricanes. The bright, turbulent storm head was able to chomp all the way around the planet. It was only when the head of the storm ran into the vortex in June 2011 that the massive, convective storm faded away. Why the encounter would shut down the storm is still a mystery.

By 28 August, after 267 days, the Saturn storm stopped thundering for good. While Cassini’s infrared detectors continue to track some lingering effects in higher layers of Saturn’s atmosphere, the troposphere – which is the weather-producing layer, lower in the atmosphere – has been quiet at that latitude.

“This thunder-and-lightning storm on Saturn was a beast,” said Kunio Sayanagi, the paper’s lead author and a Cassini imaging team associate at Hampton University in Virginia. “The storm maintained its intensity for an unusually long time. The storm head itself thrashed for 201 days, and its updraft erupted with an intensity that would have sucked out the entire volume of Earth’s atmosphere in 150 days. And it also created the largest vortex ever observed in the troposphere of Saturn, expanding up to 12 000 kilometres across.”

This storm was the longest running of the massive storms that appear to break out in Saturn’s northern hemisphere once every Saturn year (30 Earth years). The longest storm of any size ever detected on Saturn actually unfolded over 334 days in 2009 in an area known as “Storm Alley” in the southern hemisphere, but it was about 100 times smaller in area than the latest northern storm.